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Since the unit makes the noise on Microwave but not convection it is a problem it he Microwave Circuit. Probably in either the rectifier of bad mag, one of them shorted. Just looked at the date you posted this, Im sure you have fixed or replaced this by now

I would bet that the magnetron overheated, tripping the thermal cutout on the magnetron.

This can happen if the internal cooling fan is not running, or if the vents are blocked, or if the magnetron is failing and running hot, due to age in this case I would bet.

If you or someone you know can check and replace the defective parts, it would probably be economical, especially since this is a convection model.

Here are some links you
or someone you know can use (in order given) for test help, but read
the safety warnings first:http://www.gallawa.com/microtech/diode.html(if it is NOT shorted or burned looking,
it's probably fine)

It is not your key panel, but the magnetron inside the microwave that is causing you the problem. The reason it does not short out on defrost or convection is because it is not drawing as much current through the magnetron to short out.

We replaced the board behind the control panel. $130.00. If you do this make sure you can put the wires back the right way. My husband finally got it and our micro is working like brand newThanks to all who tried to help.

Typically, when the fuse blows in the microwave, the high voltage capacitor fails and becomes shorted.The door switches are also configured in such a way that if tampered with (in this case, failed switch or latch) the fuse will also blow. Check the capacitor first.